Add overview for container concept (#18090)

content/en/docs/concepts/containers/overview.md

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+---
+reviewers:
+- erictune
+- thockin
+title: Containers overview
+content_template: templates/concept
+weight: 1
+---
+
+{{% capture overview %}}
+
+Containers are a technnology for packaging the (compiled) code for an
+application along with the dependencies it needs at run time. Each
+container that you run is repeatable; the standardisation from having
+dependencies included means that you get the same behavior wherever you
+run it.
+
+Containers decouple applications from underlying host infrastructure.
+This makes deployment easier in different cloud or OS environments.
+
+{{% /capture %}}
+
+
+{{% capture body %}}
+
+## Container images
+A [container image](/docs/concepts/containers/images/) is a ready-to-run
+software package, containing everything needed to run an application:
+the code and any runtime it requires, application and system libraries,
+and default values for any essential settings.
+
+By design, a container is immutable: you cannot change the code of a
+container that is already running. If you have a containerized application
+and want to make changes, you need to build a new container that includes
+the change, then recreate the container to start from the updated image.
+
+## Container runtimes
+
+{{< glossary_definition term_id="container-runtime" length="all" >}}
+
+{{% /capture %}}
+{{% capture whatsnext %}}
+* Read about [container images](/docs/concepts/containers/images/)
+* Read about [Pods](/docs/concepts/workloads/pods/)
+{{% /capture %}}